Delay Attribution (and then payments) is not an "it's not my fault" getout. It is there to incentivise the railway to ameliorate the causes of delay as much, and in particular as QUICKLY, as they are able, rather than easing back for the rest of the day or whatever with a "well, we had a problem earlier" attitude. The quicker you get everything back up, the less attribution there is.
The particular circumstances are not really part of this. And it is up to Network Rail themselves to deal with their supplier, of electricity, just as any other rail industry supplier. Did they discuss with National Grid not being part of the load shedding when grid problems happen? Did they pay extra for this? All part of managing your suppliers. And did the TOC ensure with their train provider that trains restart easily without fitters having to be sent out?
Some things seem beyond control, trees down or flooding, but the industry knows the numbers and that this maybe impacts 0.1% of operations. What the approach is trying to prevent is this in turn impacting 10 times that number of trains because you have no effective contingency plan or anything else to restore operations pronto. Or don't bother.
Just for those defending that last comment, one reads that although Kings Cross was "closed" yesterday, and many were inconvenienced, a couple of 12-car trains did depart in the evening for Peterborough but only those "in the know", which seemingly was principally train crews, got to know about this and managed to board otherwise near-empty trains, plus a handful of passengers who didn't believe the notices.